In contemporary automotive systems it is often desirable to measure an absolute pressure at a location. For instance, it is desirable to measure a pressure across a sharp edge orifice in an EGR (exhaust gas reflow) system in order to determine flow. Often, as in this case, the media can be very harsh. Because of this adverse environment, isolation from the medium, here exhaust gas, is desirable to ensure that the sensor, typically semiconductor based, survives and functions properly over a long period of time.
Absolute pressure sensors are normally built with the vacuum inside a sensor's cavity, and have a sensed pressure applied to their top side. This side can be identified as the one that contains things for interconnection (metal wire bonding pads, wire bonds etc). As can be imagined the top side is sensitive to the chemical/physical contamination which can adversely affect operation of the sensor, causing severe degradation in sensors accuracy or even catastrophic failures.
One prior art solution to protect the sensor's top side is to use special semiconductor films (for example, nitride passivation) and outside films or gels. This kind of protection has its limitations. For example, extreme media, such as vehicle's exhaust gas, can still harm the sensing element despite being protected.
Another prior art approach to isolate the media from the sensor is using stainless steel diaphragms for sensing a pressure coupled by oil to a conventional semiconductor based pressure sensor. The stainless steel diaphragm provides the necessary isolation between the harsh media and the pressure sensor, and the oil provides the transfer of pressure to the pressure sensor. The oil medium used in this approach adds error to a pressure measurement because in the manufacturing process is difficult if not impractical to eliminate all air pockets. These air pockets add error to the pressure transfer between the stainless steel diaphragm sensing the media harsh pressure and the actual pressure sensor. Also, the oil pressure transfer performance is degraded with increasing temperature and time because of changes in oil viscosity and leakage of oil. Furthermore, using the oil filled approach is difficult to manufacture because the oil needs to be hermetically sealed between the stainless steel diaphragm and the pressure sensor.
Another prior art approach isolated the media isolated by enclosing a differential pressure sensor in the hermetically sealed vacuum can. This way a reference pressure is kept at zero whereas sensed pressure is applied from sensors back side. The back side of the sensing element, composed of the sealed silicon is not affected by the sensed media. However, this kind of solution is expensive, leak prone and not well suited for mass production.
What is needed is an improved media-isolated absolute pressure sensor, that is more accurate, easier to manufacture, low cost, and has an improved field performance over time and temperature variations.